1. Field of the Invention
The present invention relates to photoelectric converting devices for converting light energy to electrical energy and, more particularly to a photoelectric converting device using a Group III-V compound semiconductor with enhanced photoelectric conversion efficiency for converting solar light energy to electrical energy especially for use in outer space.
2. Description of the Background Art
In recent years, a greater number of multi-junction solar cells mainly including semiconductors of a Group III-V compound such as GaAs are used as solar cells for outer space of a power supply source for a space craft such as a space satellite. Such solar cells can provide greater photoelectric conversion efficiency as compared with conventional silicon solar cells which have been widely used as solar cells for outer space. As such, silicon cells are suitable for use in a small satellite or a superpower satellite.
The most popular multi-junction solar cell is of the type disclosed for example in U.S. Pat. Nos. 5,223,043 and 5,405,453. The structure of such a solar cell is shown in FIG. 20. The conventional multi-junction (two-junction) cell mainly includes a first solar cell (hereinafter referred to as xe2x80x9ca top cellxe2x80x9d) 104 of Ga1-xInxP formed on the solar light incidence side and a second solar cell (hereinafter referred to as xe2x80x9ca bottom cellxe2x80x9d) of GaAs below the top cell, which are connected by a tunnel junction 103. GaAs or Ge single-crystal wafer is used as a substrate 101. For a composition ratio of Ga1-xInxP of the top cell, x equals to 0.49 for the purpose of providing lattice matching with GaAs of the bottom cell. In this case, the lattice constants of the top and bottom cells are designed to be approximately equal to that of Ge of the substrate and to enable epitaxial growth on the Ge substrate relatively easily. Then, the bandgap Eg of the top cell is about 1.9 eV, and that of the bottom cell is about 1.4 eV. The conventional multi-junction solar cell has attained about 26% and about 22%, respectively at experimental and industrial product levels, as a result of characteristic testing using a light source as a solar light spectrum in outer space. Recently, a three-junction solar cell has been developed which has a pn junction also for a Ge substrate in addition to top and bottom cells.
To keep up with a dramatic progress in recent space development, the above mentioned photoelectric conversion efficiency is insufficient and higher conversion efficiency is desired. The above described conventional multi-junction solar cell has been developed from a GaAs solar cell formed on a Ge substrate, leading to the above described structure. In terms of solar energy efficiency, however, the combination of Ga1-xInxP and GaAs is not optimum for the following reasons.
The theoretical photoelectric conversion efficiency of a solar cell having two pn junctions is described for example in an article in IEEE Transactions on Electron Devices. ED-34, p257. The article shows a relationship between the expected value of photoelectric conversion efficiency and a range of the bandgaps of top and bottom cells based on matching of the bandgaps of top and bottom cells and incident light spectrum. In practically manufacturing a solar cell, lattice matching between top and bottom cells as well as between the bottom cell and the substrate must be achieved to provide a high-quality epitaxial layer. FIG. 21 shows a relationship between a lattice constant and a bandgap energy for various semiconductor materials. Based on the above mentioned article, FIG. 21 shows bandgap ranges U and L respectively for the top and bottom cells to achieve a conversion efficiency of at least 30% with respect to a solar light spectrum (AMO) in outer space. The graph shows that the combination of the materials used for the above described conventional multiunction solar cell, i.e., the combination of Ga1-xInxP and GaAs, merely provides the photoelectric conversion efficiency of no more than 30%.
An object of the present invention is to provide a photoelectric converting device capable of providing increased photoelectric conversion efficiency by optimizing a combination of materials for top and bottom cells.
A photoelectric converting device of the present invention is provided with first and second pn junctions. The first and second pn junctions are substantially formed in semiconductors respectively represented by (Al1-yGay)1-xInxP and Ga1-zInzAs.
To achieve a convention efficiency of at least 30%, it has been said that at least the following conditions must be met.
(a) Optimization of a combination of a material for a top cell (hereinafter referred to as xe2x80x9ca top cell materialxe2x80x9d) and a material for a bottom cell (hereinafter referred to as xe2x80x9ca bottom cell materialxe2x80x9d).
(b) Lattice matching between the top and bottom cell materials.
(c) Lattice matching between the bottom cell and substrate materials.
(d) Matching of thermal expansion coefficients between a layer and substrate materials.
However, it is difficult to find a combination of semiconductor materials which satisfy all of these conditions and which are still inexpensive. The extensive study of each of the above conditions conducted by the present inventors have confirmed that the above conditions (a) and (b) are indispensable to provide a conversion efficiency of at least 30%.
However, the following finding was also obtained. Namely, lattice matching between the bottom cell and substrate materials are not very important, and lattice mismatching of at most about 4% can still provide a layer with good crystallinity by a crystal growth technique. (This condition, an alleviation of (c), will be hereinafter represented as (cxe2x80x2)).
In addition, the following finding was obtained. Matching of thermal expansion coefficients between the layer and the substrate materials is not extremely important either and, as long as the thermal expansion coefficient of the layer is at most that of the substrate, the problem of cracks to the layer caused by the difference in thermal expansion coefficient can be avoided. (This condition, an alleviation of (d), will be represented by (dxe2x80x2)).
The above described study confirmed that, as materials satisfying the conditions (a), (b), (cxe2x80x2) and (dxe2x80x2), semiconductors represented by (Al1-yGay)1-xInxP and Ga1-zInzAs are respectively effective for top and bottom cells. (Al1-yGay)1-xInxP and Ga1-zInzAs could be found mostly because of the conditions (cxe2x80x2) and (dxe2x80x2). With use of the structure of the present invention, all of the above conditions (a), (b), (cxe2x80x2) and (dxe2x80x2) can be satisfied. Consequently, a photoelectric converting device with a conversion efficiency of at least 30% can be achieved. Note that, in chemical composition representation, an element C occupies only x (xe2x89xa61.0) of a site of C in a crystal grating with a chemical formula CP, while an element B occupies the remaining site of 1-x in the case of B1-xCxP including B, C, and P. For (A1-yBy)1-xCxP, B occupies only y(xe2x89xa61.0) of a site of B in B1-xCxP, while A occupies the remaining 1-y For a Group III-V compound semiconductor of the present invention, InP, InAs, GaAs, GaP or the like generally has a zinc blende crystal structure. The zinc blende crystal structure is similar to a diamond structure of a semiconductor of Group IV like Ge, Si. In the photoelectric converting device of the present invention, composition ratio z, x and y of semiconductors Ga1-zInzAs and (Al1-yGay)1-xInxP desirably fall within 0.11 less than z less than 0.29, x=xe2x88x920.346z2+1.08z+0.484 and 131z3xe2x88x9266.0 z2+9.17z+0.309 less than y less than 28.0z3xe2x88x9224.4z2+5.82z+0.325, respectively.
Specifically, the structure optimizes bandgap energies of the top cell and bottom cell materials. The present inventors have conducted calculations of lattice constants and bandgap energies for these semiconductors to find an optimum combination of (Al1-yGay)1-xInxP and Ga1-zInzAs. FIG. 1 is a graph showing a range A of bandgap energies of top and bottom materials providing a photoelectric conversion efficiency of at least 34%. Referring to FIG. 1, the abscissa and ordinate respectively represents the bandgap energies of the top and bottom cell materials. In this graph, a region A, which is expected to provide a conversion efficiency of at least 30%, is represented as a region enclosed by a single closed curve. In FIG. 1, segments which are parallel to the abscissa show a relationship between the bottom and top cell bandgaps when the top cell material is a compound crystal with a given bottom cell material. Since the top cell is a compound crystal, the bandgap also has a range in accordance with a range allowing the compound crystal, which is represented as a segment. Shown on the right side of each segment are two semiconductor materials, i.e., a semiconductor material for the bottom cell and a compound crystal thereon. For the bottom cell listed therein, lattice mismatching to Ge is indicated as a percentage. For example, Ga0.29In0.71Pxe2x80x94Al0.30 In0.70P on Ga0.77In0.23As (1.62% greater than Ge) represents that the top cell is a compound crystal of Ga0.29In0.71 Pxe2x80x94Al0.30In0.70P and the bottom cell is Ga0.77In0.23As. In addition, it indicates that the lattice constant of bottom cell Ga0.77In0.23As is greater by 1.62% than that of GE. The ranges of(Al1-yGay)1-xInxP and Ga1-zInzAs, respectively forming the top and bottom cells falling within region A of a conversion efficiency of at least 34%, are as follows.
z: a composition ratio z Ga1-zInzAs for the bottom cell falls within 0.11 less than z less than 0.29.
x, y: composition ratio x and y of (Al1-yGay)1-xInxP for the top cell are respectively x=xe2x88x920.346z2+1.08z+0.484 and 131z3xe2x88x9266.0z2+9.17z+0.309 less than y less than 28.0z3xe2x88x9224.4z2+5.82z+0.325, given that z is within the above mentioned range. The range of x is as shown in FIG. 2, according to composition ratio z of Ga1-zInzAs for the bottom cell. The range of y is as shown in FIG. 3 according to composition ratio z of Ga1-zInzAs for the bottom cell.
It is expected that a conversion efficiency of at least 34% is attained if composition ratios x, y and z of the top and bottom cell materials are within the above range. Further, if x, y and z are in the above range, lattice mismatching to Ge can be limited to below 2% if the substrate is of Ge. For a thermal expansion coefficient, three materials are very close to one another as Ge: 5.5xc3x9710xe2x88x926/K, Ga1-zInzAs: 5.8xc3x9710xe2x88x926/K and (Al1-yGay)1-xInxP: 4.8xc3x9710xe2x88x926/K, cracks would not be caused to or run through a layer.
The present invention is characterized in that a new material is used as a semiconductor material forming a solar cell by alleviation of the condition for lattice matching to the substrate. Namely, for the top cell, a new semiconductor is obtained based on the concept of compound crystal. In other words, the same crystal structure is used but elements forming the crystal are changed, or composition ratios of the elements are changed to provide a novel semiconductor. In general, for a compound semiconductor, it is well known that a compound crystal is obtained by mixing materials having the same crystal structure, a compound crystal with an intermediate characteristic in terms of lattice constant, bandgap energy or the like according to the ratio. Such a compound crystal is practically used for a device like an LED (Light Emitting Diode), laser diode or the like. In this case, the amount of added materials is not to the extent of mere doping of impurities, but is large enough to cause a composition change involving changes in crystal lattice constant, bandgap and the like. In this respect, the above described semiconductors Ga1-zInzAs and (Al1-yGay)1-xInxP manufactured based on the concept of compound crystal are novel.
Now, the difference between the present invention and the above mentioned United States Patents, of which concept the present invention is based, will be described in detail.
(1) The difference from U.S. Pat. No. 5,223,043
The above mentioned United States Patent discloses the following three combinations of materials for a two-junction solar cell.
(A) A combination of top cell GaxIn1-xP (0 less than x less than 0.5) and bottom cell GaAs
(B) A combination of top cell GaxIn1-xP (x=0.51xc2x10.05) and bottom cell GaAs
(C) A combination of top cell GaxIn1-xP (0 less than x less than 0.5) and bottom cell Gax+0.5In0.5-xAs (0 less than x less than 0.5)
Among the above combinations, (A) and (B) are on the premise that a layer is lattice-matched to a Ge substrate as stated previously. On the other hand, the layer of the present invention does not have to be necessarily lattice-matched to the Ge substrate as defined in (cxe2x80x2). The material of the top cell used for the photoelectric converting device of the present invention is different from that of the above mentioned United States Patent. Namely, Al0.15Ga0.15In0.7P, a typical top cell material for the photoelectric converting device of the present invention, has 15% of Al, deviating from any of (A), (B) and (C) of the aforementioned United States Patent. In other words, in the present invention, a high conversion efficiency is attained by appropriately setting the bandgaps of the top and bottom cells with use of (Al1-yGay)1-xInxP including Al for the top cell. To attain such a high conversion efficiency, a semiconductor material including at least a prescribed amount of Al as a material of the top cell must be used.
(2) The difference from U.S. Pat. No. 5,405,453
The above United States Patent discloses the following two combinations of materials for the two-junction solar cell.
(D) A combination of top cell (Ga, In) P (typically Ga0.49In0.51P) and bottom cell GaAs.
(E) A combination of top cell (Al, In) P (typically Al0.55In0.45P) and bottom cell GaAs.
The above (D) and (E) are combinations on the premise of lattice matching to a Ge substrate, which is basically different from the present invention in solar cell designing. In addition, both of the top cell and bottom cell are different from the present invention in materials used.
(3) Others
Other disclosure (Technical Digest of the International PVSEC-11, Sapporo, Hokkaido, Japan, 1999, p593-594) discloses the following combination.
(F) A combination of top cell In0.49Ga0.51P and bottom cell In0.01Ga0.99P
The combination (F) provides for lattice matching to the Ge substrate with use of In0.01Ga0.99P obtained by including In in GaAs by 1% to correct slight lattice mismatching between GaAs, a conventional bottom cell material, and Ge. Thus, the combination (F) is basically different from that of the solar cell of the present invention in designing. Further, both of the top and bottom cells are different from that of the solar cell of the present invention in materials used.
The present invention needs not be lattice-matched to the substrate as compared with the prior art of (A) to (F). In the present invention, a novel material (Al1-yGay)1-xInxP is used for the top cell, and top and bottom cells both have set ranges of composition ratios of the materials used.
Desirably, the photoelectric converting device has a tunnel conjunction of (Al1-yGay)1-xInxP and Ga1-zInzAs, for example.
The tunnel junction has a p+ n+ junction which is highly doped for electrically connecting the top and bottom cells. With such a structure, incident light with high energy at the top cell is partially converted to electrical energy, and light energy decreased by an amount converted at the top cell is partially converted to electrical energy at the bottom cell. Namely, tandem conversion is performed. In addition, electrical energy would be hardly lost after the conversion because of the tunnel junction. Thus, electrical energy can be used by the photoelectric converting device with higher conversion efficiency.
The photoelectric converting device of the present invention has, for example, a buffer layer between a layer including first and second pn junctions formed on a substrate and the substrate, where the thermal expansion coefficient of the buffer layer may be at least that of the layer immediately above the buffer layer.
With such a structure, cracks can be restrained only in the buffer layer in case of temperature change from high temperature to low temperature when forming the layer by MOCVD (Metal Organic Chemical Vapor Deposition). Thus, cracks are not caused to or run through the layer.
In the photoelectric converting device of the present invention, the thermal expansion coefficient of the substrate is desirably smaller than that of the layer immediately above the buffer layer.
With such a structure, the buffer layer can more reliably prevent cracks. The lattice constant of the buffer layer material is close to those of the layer and substrate. Further, in the converting device where the buffer layer material has a thermal expansion coefficient at least equal to or smaller than that of the layer immediately above, the lattice constant of the buffer layer material is desirably matched to that of the layer immediately above the buffer layer. More specifically, it is desirable that the buffer layer material substantially includes GaAs1-wSbw (0.29 less than w less than 0.33), for example. The above mentioned composition ratio w can be appropriately selected within the range of 0.29 less than w less than 0.33 to provide lattice matching or slight lattice mismatching according to the value of composition ratio z (0.11 less than z less than 0.29) of Ga1-zInzAs.
In the photoelectric converting device of the present invention, for example, it is desirable that a layer including first and second pn junctions is formed on at least one of GaAs, Ge, and Si single crystal substrates.
With such a structure, a layer with good crystallinity is formed to easily provide a photoelectric converting device with higher conversion efficiency. In addition, the Ge single crystal of the substrate limits lattice mismatching to the substrate within 2%. Thus, a high-quality layer is produced by epitaxial growth, and still higher conversion efficiency can be achieved if a pn junction is also formed for the Ge.
In the photoelectric converting device of the present invention, for example, a layer including first and second pn junctions formed on the substrate can be provided above an Si1-xGex compound crystal layer on the Si single crystal substrate.
With such a structure, lattice mismatching is alleviated and a layer can be formed with excellent crystallinity. In addition, the photoelectric converting device with higher conversion efficiency can be obtained with an inexpensive substrate.
In the photoelectric converting device of the present invention, for example, a pn junction can be further formed at an upper layer portion of the substrate where the layer including the first and second pn junctions is formed.
Such a structure provides for effective usage of light and enhanced photoelectric conversion efficiency.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.